The experiments described in this proposal are designed to: (1) determine whether the anti-nociceptive effect of intracranially administered morphine is specific with respect to central locus; (2) determine to what extent the anti-nociceptive effect of intracranially administered morphine may depend with respect to the nature of the noxious stimulus employed; and (3) examine the similarities/dissimilarities of the analgesias induced by intracranially administered morphine and focal brain stimulation. The effects of chlorpromazine, pentobarbital, morphine and naloxone administered intracranially via a 35 guage cannula into specific brain loci will be evaluated employing several analgesiometric tests and noxious stimuli. Cannulas and focal brain stimulating electrode-units (cannodes) will be permanently implanted in the periaqueductal central gray matter, lateral hypothalamus, septal and caudate areas in rats for purposes of testing drug effects and inducing analgesia electrically. The standard hot-plate and tail flick analgesiometric tests, firm pinching and pin-pricks applied to the extremeties and noxious intracranial trigeminal nerve and optic tract stimulation will be tested against all drugs and focal brain stimulation induced-analgesia at all brain sites. The purpose of these experiments is to elaborate on the central site(s) at which analgesic agents exert their antinociceptive action.